1. Field of the Invention
This invention relates to a pressure-sensitive recording paper. More particularly, it relates to a pressure-sensitive recording paper using a compound, in which two alkyl-substituted benzene nuclei are connected with each other through the unit --C.sub.n H.sub.2n), as a solvent for a color former: ##STR2## wherein n is an integer of from 1 to 8, R and R' each is an alkyl group having one to eight carbon atoms or a hydrogen atom, p and q each is the number of alkyl groups, p+q being an integer of 1-3, and wherein R and R' may be the same.
2. Description of the Prior Art
The object of the present invention is to obtain a pressure-sensitive recording paper having no unpleasant smell and toxicity, and capable of forming a color image having a higher density without giving a color fog.
Usually, as pressure-sensitive recording paper, there are those comprising so-called upper paper prepared by dissolving a substantially colorless compound (hereinafter referred to as a "color former") in an organic solvent, encapsulating it, and then coating the capsules onto a support; lower paper prepared by coating a color-developing material (hereinafter referred to as a "color developer"), which is capable of forming colored images, onto another support; and, in some cases, middle paper prepared by coating capsules containing a color former onto one side of a support and a color developer onto the other side thereof, or those containing said capsules and a color developer onto the same side of a support.
In the case of the combination of upper and lower paper, or upper, middle and lower paper, color is developed by locally pressing in such a way that the capsule layer comes into contact with the color developer layer to decompose the capsules located at the pressed part and to cause reaction between the color former and the color developer. When a pressure sensitive recording paper having capsules and a color developer agent on the same surface thereof is used, color will be developed by pressing in the same way. As the color developer, active clay materials, such as acid clay, active zeolite and bentonite, or organic acidic materials, such as succinic acid, tannic acid, gallic acid, pentachlorphenol and phenol resin are generally used. As a color former for a pressure sensitive recording paper, malachite green lactone, benzoyl leucomethylene blue, crystal violet lactone, rhodamine B lactom, 3-dialkylamino-7-alkylfluorans, 3-methyl-2,2'-spirobi(benzo[f]chromene), etc. are used.
The conditions a solvent dissolving a color former for pressure sensitive recording paper should satisfy are:
a. to have enough solubility to dissolve a necessary amount of color former, PA1 b. to have a high boiling point so as not to vaporize in a heat-drying process and in a place of elevated temperature, PA1 c. not to be eluted on encapsulation, PA1 d. not to desensitize or prevent the color formation on the lower paper, PA1 e. not to provide changes, such as decomposition of the color former, color formation, etc., PA1 f. to have a low viscosity so that the effusion thereof from inside the capsules can be freely done on breakage of the capsules, and to have a small rise in viscosity, even at low temperature, PA1 g. to have no unpleasant smell, PA1 h. to have little toxicity to human beings and animals,
and the like. Among these, conditions (a)-(e) are especially of importance and, if one of these conditions is not satisfied, the solvent cannot be used as a solvent for a color former.
In fact, however, solvents satisfying conditions of said (a)-(h) have so far not been found. Therefore, as things are, those prepared by mixing several kinds of solvents in a suitable ratio are used.
That is, ethers do not satisfy the conditions of (a) and (d), alcohols do not satisfy the conditions of (a), (c) and (d), parafins do not satisfy the conditions of (a), ketones, esters, olefins and amines do not satisfy the conditions of (d), and organic acids to not satisfy the conditions of (c), and usual aromatic hydrocarbons do not satisfy the conditions of (a), (g) and (h). Chlorinated diphenyl now used as a solvent for pressure sensitive recording paper, nearly satisfies the conditions of (a) to (e). It has, however, defects in that, at a low chlorination degree, it has a peculiar unpleasant smell, while it has low viscosity and that, at high chlorination degree, it has high viscosity, and does not have much of an unpleasant smell. That is, the viscosities of low chlorinated trichlorodiphenyl and tetrachlorodiphenyl are comparatively low. However, they have a peculiar unpleasant smell. Highly chlorinated pentachlorodiphenyl and hexachlorodiphenyl have considerably reduced unpleasant smell, but their viscosity conspicuously increases on the other hand. Hexachlorodiphenyl has no fluidity at a room temperature. Therefore, the pressure-sensitive recording paper wherein low chlorinated trichlorodiphenyl and tetrachlorodiphenyl are used, has the defect of having an unpleasant smell. The pressure sensitive recording paper wherein highly chlorinated pentachlorodiphenyl and hexachlorodiphenyl are used, has the defect in that the effusion thereof is difficult to be freely done due to their high viscosity and that sufficient color density cannot be obtained, while they have less of an unpleasant smell. The pressure-sensitive recording paper wherein the mixture of chlorinated diphenyl with a low chlorination degree and the same with high chlorination degree is used, is a little more improved in the smell than that wherein chlorinated diphenyl with low chlorination degree is used separately and, in the density of the color formed, the former is a little more improved than that wherein highly chlorinated diphenyl is used independently, but the former has still considerable unpleasant smell and the density of color formed is not sufficient. In addition, chlorinated diphenyl is slightly decomposed by light to form hydrogen chloride. Accordingly, the capsule coated sheet wherein chlorinated diphenyl is used has the defect that, when it is exposed to light for a long time, the generated hydrogen chloride will be reacted with the color former to cause colored fog. Furthermore, this capsule coated sheet wherein the colored fog took place has less color developing ability onto the lower paper, and a sufficient density of the color formed cannot be obtained. Besides, chlorinated diphenyl has appreciable toxicity to human beings and animals. Therefore, the conventional pressure sensitive recording paper wherein chlorinated diphenyl is used, has had the defects that it has unpleasant smell, that sufficient density of the color formed cannot be obtained, that when exposed to light for a long time, colored fog on capsule coated sheet and the lowering of the color developing ability thereof to the lower paper take place, and that it has a problem in its toxicity.